How to check the ignition (spark) coil

The ignition coil is one of the most important elements in the ignition circuitry. Although this device is quite simple there are only a few resources written about its functionality checks. My friend Frantisek has posted a short article about how to do this by using just a multimeter.

I have extended Frantisek’s article with step by step instructions and pictures. First what we need to know is the coil’s polarity. Coils are usually somehow marked by the producer already. A red or embossed point near the electrical positive (+ plus) terminal is widely used (see Trojan’s coil below). Some of the coils are unfortunately unmarked as we can notice on the Wilco coil below.

Top down: Wilco, Trojan and Smith ignition coils

Our first check is to “Find/check coil’s polarity”. Study the topmost picture first. There is a schematic ignition wiring diagram. Notice the coil itself, especially its primary (thick line, less turns, marked as “B-C”) and secondary (thin line, more turns, marked as “B-A”) windings. Letter “A” stands for high tension point that leads directly to the spark plug. I must notice that the malfunctioned/broken coil will not pass the following tests.

Now let’s assume that both primary and secondary windings share the positive (+ plus) terminal on the coil’s right side (see picture below). Connect DC voltage source (1 – 1,5V) to the coil’s terminals based on our polarity assumption. Set the multimeter to the “DC 20V” range and then connect the negative probe to the coil’s negative terminal (on the left side during this step) and the positive probe to the high tension terminal. We found out the polarity is wrong, the multimeter shows 0.00V in this scenario.

Wrong coil polarity – notice the voltage

The next step is quite simple. Change the polarity of the DC voltage source and multimeter accordingly. Now we assume that the shared positive terminal is on the coil’s right side (see picture below). As we can see, the multimeter reads 0.89V. It means the polarity is: positive (+ plus) terminal is on the left side of the coil and negative (- minus) on the right side and our coil passed the functionality test too. Notice the measured DC voltage is lower than the terminal voltage of the DC source. This is caused by the overall internal electrical resistance of the coil.

Proper coil polarity means this coil passed the test – notice the voltage

The second scenario’s goal is to check/measure both primary and secondary windings for electrical resistance or malfunction one or both. Let’s check the primary winding first. Set the multimeter to the “Ω 200”. Connect probes to both coil terminals (see picture below). We could expect to measure electrical resistance in a range of Ω (ohm). In our case, it is about 5,2Ω. The primary winding is alright.

Primary winding electrical resistance is 5,2Ω

And lastly, let’s check the secondary winding. Do not forget to set the multimeter to the “Ω 20k” and connect probes to the secondary coil terminals as shown in the picture below. The measured value is 3,74kΩ, which is in the usual range of coil in a good working condition.

Secondary winding electrical resistance is 3,74kΩ

As you can see we were able to find out the electrical polarity of this coil and confirm its working condition. This mini how-to is as simplified as anyone can perform these tests by using just basic equipment. Happy testing/finding 🙂

1 Response

  • Benny Limanhadi // // Reply

    Thank you for sharing your knowledge. This (sparky engines) is new for me after more than 50 years in this hobby.
    I will try mine.

    Keep sparky alive.

    Benny Limanhadi

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